The present invention generally relates to devices and methods for detecting and/or measuring substances. More particularly, this invention relates to a gas sensing method and instrument capable of detecting the presence of a gas at low levels within an environment, and accurately measuring the concentration of the gas in the environment.
Gas leak detectors are widely used in the mining industry and by utility company personnel, first responders, and others to detect the presence of potentially harmful or dangerous gases, a notable but nonlimiting example being hydrocarbon gases such as methane. Gas leak detectors typically use a thick-film metal oxide semiconductor sensor whose metal oxide film is reactive to the targeted gas and when reacted exhibits a change (usually a drop) in electrical resistance. The response is nonlinear relative to the amount of targeted gas present, and as such typical gas leak detectors are very sensitive at low level concentrations (e.g., up to about 10,000 ppm), but lose their sensitivity at higher concentrations (generally at a few percentages of gas concentration) at which point the output is said to encounter signal saturation. An example is the FIGARO 2600 series methane gas sensor model TGS2611, commercially available from Figaro Engineering Inc.
FIG. 1 represents by example a gas detector 10 that utilizes a nonlinear sensor 12 of the type described above. The sensor 12 is interfaced with audio circuitry 20 containing an audio device 24 for producing an audible “tick” whose rate or frequency is in proportion to the gas concentration sensed by the sensor 12. As used herein, an “audible tick” refers to a variable repetition rate of audio pulses, each, for example, approximately 250 msec in duration, to which a human ear is very responsive. The tick rate alerts the user to the presence of a gas to which the sensor 12 is sensitive and, prior to the onset of signal saturation, the relative amount of gas. As evident from FIG. 1, the sensor 12 interfaces with the circuitry 20 by functioning as part of a resistive (voltage) divider circuit that contains the sensor 12 and a resistor 14 connected in series to a suitable DC or AC voltage source 16. The resistive divider circuit is shown connected to a linear amplifier 22, the audio device 24, and a speaker 26 (or other audible sound-generating device), which cooperate to convert voltage to an audible sound. The audio device 24 is represented as a voltage controlled oscillator (VCO)/pulse generator, whose repetition rate is in proportion to gas concentration sensed by the sensor 12 and drives the speaker 26. Gas detector circuits of the type represented in FIG. 1, including its circuit components and their electrical properties, are well known in the art and therefore will not be discussed in any further detail here. The electrical values of the components are indicated in FIG. 1 for reference purposes only.
Because of their nonlinear response characteristics, semiconductor-type gas sensors are not well suited for performing actual measurements of gas concentrations. In contrast, pellistors and other types of sensors, e.g., infrared (IR) sensors, are more suitable for actual measurement of gas concentration because they have essentially linear responses. As well known in the art, the detecting element within a pellistor comprises catalyst-loaded pellets or beads of a ceramic material whose resistance changes with temperature. The catalyst is usually a palladium and/or platinum alloy, which oxidizes organic gas molecules to yield largely water vapor and carbon dioxide. The resulting change in temperature from the heat of oxidation causes a change in resistance in the ceramic material, which in turn is then measured and related to the quantity of the targeted gas present in the environment or other sample. An example of a pellistor sensor is the model CH-D3 combustible gas pellistor commercially available from Alphasense Ltd. Because their output response is linear, pellistor-type sensors are not ideally suited for use as leak detectors requiring high sensitivity at very low gas concentrations (e.g., below a few percentages of gas concentration).
In view of the above issues, combinations of nonlinear sensors and linear sensors are widely used when the desire is to provide a device capable of both gas detection and gas concentration measurement. However, it would be desirable to avoid the cost of combining both technologies in a single instrument.